Solar concentration without mirrors

Photonics SpectraMar 2012
Mirror-free thermophotovoltaic devices could someday make a much simpler and less expensive system to concentrate sunlight. The goal is to prevent heat from escaping the thermoelectric material by using a photonic crystal – essentially, an array of precisely spaced microscopic holes in a top layer of the material.

By concentrating the sunlight thermally – capturing it and reflecting it back into the material – the device could absorb as much heat as a standard black object, yet not reradiate much of the heat, suggests Peter Bermel, a scientist at MIT’s Research Laboratory of Electronics.

A diagram of an angle-selective solar thermophotovoltaic system. Courtesy of P. Bermel et al, Nanoscale Research Letters 2011 6:549.
Infrared radiation from the sun could enter the device through the holes on the surface, but the reflected rays would be blocked when they try to escape; this is much like the Earth’s greenhouse effect. The blockage is achieved with a precisely designed geometry that allows only those rays that fall within a very tiny range of angles to escape, while the rest stay in the material and heat it up – theoretically, to very high temperatures.

In direct sunlight, an ordinary dark-colored, light- and heat-absorbing material can’t get much hotter than boiling water because the object reradiates heat almost as fast as it absorbs it; power generation requires higher temperatures than that. By concentrating sunlight with parabolic mirrors or a large array of flat mirrors, much higher temperatures are possible, but at the expense of a much larger and more complex system.

Bermel said that such a system “at large scale, is efficient enough to compete with more conventional forms of power. This is an alternative to concentrators.”

The efficiency of ordinary solar-energy-harnessing systems is around 10 percent, but the new theoretical material could achieve 32 to 36 percent. It is important to note that an increase in efficiency of even 1 percent is considered significant.

The system is simple to manufacture, using standard chip-fabrication technology, Bermel said. By contrast, the mirrors used for traditional concentrating systems require extremely good optics, which are expensive.

The advance envisions the use of existing light-absorbing material to create a photonic structure that preferentially emits light in a direction and wavelength range optimal for photovoltaic conversion, said Jason Fleischer, an associate professor of electrical engineering at Princeton University in New Jersey, who was not involved in this work.

Doing so, Fleischer said, “increases the efficiency significantly beyond classical predictions based on unconcentrated sunlight, enabling a small device to generate as much electricity as a conventional one that is much larger.”